Probing clumpy stellar winds with a neutron star

Context. INTEGRAL, the European Space Agency's gamma ray observatory, tripled the number of super-giant high-mass X-ray binaries ( sgHMXB) known in the Galaxy by revealing absorbed and fast transient ( SFXT) systems. Aims. In these sources, quantitative constraints on the wind clumping of the massive stars could be obtained from the study of the hard X- ray variability of the compact accreting object. Methods. Hard X- ray flares and quiescent emission of SFXT systems have been characterized and used to derive wind clump parameters. Results. A large fraction of the hard X- ray emission is emitted in the form of flares with a typical duration of 3 ks, frequency of 7 days and luminosity of 10(36) erg/ s. Such flares are most probably emitted by the interaction of a compact object orbiting at similar to 10 R-* with wind clumps (10(22-23) g) representing a large fraction of the stellar mass- loss rate. The density ratio between the clumps and the inter- clump medium is 10(2-4) in SFXT systems. Conclusions. The parameters of the clumps and of the inter- clump medium, derived from the SFXT flaring behavior, are in good agreement with macro- clumping scenario and line driven instability simulations. SFXT have probably a larger orbital radius than classical sgHMXB.